1. Field of the Invention
The present invention relates generally to a distributor-type fuel injection pump for use with an internal combustion engine such as a diesel engine for automobiles, and more particularly to such a fuel injection pump so constructed as to perform the preliminary fuel injection in advance of the main injection.
2. Prior Art
Preliminary fuel injection has been put into practice to keep the inside atmosphere in the engine cylinder in a more ignitable condition before the next following main fuel injection, thereby lightening a sudden and violent combustion upon the main fuel injection, which would otherewise cause an excessive combustion noise. Known preliminary fuel injection systems disclosed in Japanese Patent Laid-open Publication Nos. 56-56982 and 57-173552 comprise a plunger having defined therein main and preliminary fuel injection ports communicating with each other for effecting a preliminary fuel injection when the preliminary fuel injection port is brought into alignment with a distribution passage in advance of the main fuel injection port. However, such preliminary fuel injection is always effected in disregard of the engine speed (revolutions per minute) with the result that the amount of fuel to be delivered from the main fuel injection port decreases, thus causing a reduction in engine power.
According to an apparatus shown in Japanese Patent Laid-open Publication No. 54-126820, a timer is used to control the preliminary fuel injection timing such that the preliminary fuel injection takes place within a predetermined range of engine speed in dependence on the engine r.p.m. An angle defined by and between main and preliminary fuel injection ports is properly set so as to enable the preliminary fuel injection to be effected until the fuel injection advance angle reaches a predetermined threshold value which is variable under the control of a timer. The timer performance varies with the engine performance embodying the fuel injection pump. However, as shown in FIG. 9, it is generally set to retard or maintain the advance angle Ta at the reference value 0 until the engine speed reaches a predetermined value Nc which is excessively larger than an idling speed range; thereafter the advance angle Ta increases toward its maximum value. With this arrangement, the preliminary fuel injection is effected in a relatively wide range of engine speed before the angle is advanced. Since unpleasant combustion engine noise is practically observed when the engine operates near an idling speed, the prior apparatus also causes an undesirable reduction in engine power.